People in Britain could be able to see their Covid-19 vaccination status on an app designed by the National Health Service.
The UK government is reviewing the ethical and economic implications of "Covid status certification" as part of its plan to ease restrictions in England.
The review, led by Cabinet Office Minister Michael Gove, could pave the way for the introduction of Covid passports in the domestic economy.
It is understood ministers will consider the future role of the NHS app as part of the review.
The app could be used as a health passport to show inoculation status and negative test results.
It is separate from the NHS Covid-19 app, which has been downloaded more than 21 million times since last year and is used mainly for contact tracing.
The generic NHS app already allows people to view their medical records, including vaccinations.
The government is looking at whether vaccine certificates can help restart major events, while the review will also consider how such proof might be used by employers or venue owners.
The plans come as the UK uses its presidency of the G7 to push for an internationally recognised system of vaccine passports that could allow world travel to resume.
The government said it would work with other countries through the World Health Organisation, the G7 and other groups on “a clear international framework with standards that provide consistency for passengers and industry alike".
"Introducing such a system also needs to be fair and not unduly disadvantage people who have yet to be offered or gain access to a vaccine," the government said in a document outlining its plans to lift restrictions.
“That being the case, the government does not expect this solution to be available quickly and restrictions like those in place around the world are likely to continue for the near future.”
UK Prime Minister Boris Johnson said there were "deep and complex issues" with using vaccine passports or certificates to show someone's Covid-19 status in the domestic economy.
Mr Johnson said the UK had "never thought in terms of having something that you have to show to go to a pub or a theatre".
“We can’t be discriminatory against people who for whatever reason can’t have the vaccine,” he said.
“There might be medical reasons why people can’t have a vaccine. Or some people may genuinely refuse to have one.
"I think that’s mistaken, I think everybody should have a vaccine, but we need to thrash all this out.”
Campaign group Liberty said vaccine passports could create a two-tier society.
"Ministers have flip-flopped on the idea of immunity passports for months before quickly backing out again when faced with immense backlash," Liberty's policy and campaigns chief Sam Grant said.
“Renaming them ‘status certificates’ does nothing to address the fact that they would create a two-tier society, where some people can access support and freedoms while others are shut out – with the most marginalised among us hardest hit.”
Dr Mike Tildesley, who advises the government on infectious diseases, said resistance to inoculation remained high in parts of the UK, raising the possibility of discrimination against people from areas where vaccine acceptance was low.
“This is a real concern," he told the BBC. "We may end up in a situation where we have the ‘vaccine rich’, as it were, who are able to access the vaccine, who have taken up the vaccine and are at much lower risk, and maybe the people in society who have not taken up the vaccine.
“Potentially, these individuals could be clustered in particular parts of the country and there is increased risk there.”
What is a vaccine passport and how does it work?
A vaccine passport or certificate is a document that will allow you extra freedom under coronavirus restrictions.
The document shows whether a person has been inoculated against Covid-19.
Vaccine passports are being considered to allow travellers to go overseas without having to quarantine at their destination.
They could also be used to allow someone entry into a major event with large crowds such as a concert.
